1. Field of the Invention
The present invention relates to a fixing apparatus mounted on an image forming apparatus such as an electrophotography printer or an electrophotography copying machine, a roller for a fixing apparatus and a flexible sleeve for the fixing apparatus used in the fixing apparatus. In addition, the present invention also relates to methods of manufacturing the roller for the fixing apparatus and the flexible sleeve for the fixing apparatus.
2. Description of the Related Art
A printer or a copying machine of the electrophotography type includes an image fixing apparatus for heat-fixing an unfixed toner image formed on a recording material thereonto. As a type of the fixing apparatus, there is a heat roller type including a fixing roller (roller for the fixing apparatus), a heat source such as a halogen heater disposed inside the fixing roller, and a pressure roller for forming a fixing nip portion together with the fixing roller. In addition, there are various types of fixing apparatus such as an on-demand type including an endless belt (hereinafter, also referred to as flexible sleeve for the fixing apparatus, fixing sleeve, or fixing film), a ceramic heater for contacting with an inner surface of the endless belt, and a pressure roller for forming a fixing nip portion together with the ceramic heater through the endless belt.
The fixing roller includes a roller core bar having high stiffness, and a rubber layer and a fluororesin layer formed on the core bar. This fixing roller has an advantage that high pressure can be applied to the recording material because of its high stiffness, and hence the fixing roller can be used suitably for a printer or a copying machine for high speed printing. In contrast, the fixing sleeve includes a resin film such as polyimide or a metal film such as a stainless steel, on which a rubber layer and a fluororesin layer are formed. The fixing sleeve has an advantage in that thermal capacity thereof can be reduced. The rubber layer is made of silicone rubber or the like, and has a role of improving its property of following pits and projections of the recording material or the toner image so that heat can be conducted uniformly to the toner image for achieving high image quality. The fluororesin layer has a role of preventing the toner having cohesiveness from sticking to and remaining on the surface of the fixing roller or the surface of the fixing sleeve.
Recently, in order to achieve high speed printing, to save power, and to improve image quality as for the printer, each type of the fixing apparatus including the heat roller type using the fixing roller and the on-demand type using the fixing sleeve is required to have higher efficiency for conducting heat to the recording material. Therefore, the fixing roller and the fixing sleeve are required to have high thermal conductivity, small thermal contact resistance with the toner image, i.e., high heat conduction efficiency from the heater to the toner image.
Therefore, the rubber layer is made of silicone rubber having high thermal conductivity and is formed to have an appropriate thickness. Thus, surfaces of the fixing roller and the fixing sleeve are adapted to have flexibility, and good property of following pits and projections of the recording material or the toner image is secured. As a result, the thermal contact resistance between the fixing roller and the recording material, as well as the thermal contact resistance between the fixing sleeve and the recording material can be reduced. In contrast, the fluororesin of the top surface layer has larger coefficient of elasticity and lower thermal conductivity than the silicone rubber. Therefore, the fluororesin layer can be formed as thin as possible for a purpose of securing the property of following and a purpose of improving the thermal conductivity. If the heat conduction efficiency from the heater to the toner image is high, a toner image can be fixed onto a surface of a recording material securely by the heat-fixing even in a printer for supporting high conveying speed of recording material. Therefore, it is possible to form an image having high quality with little density reduction or image loss even if the toner image is rubbed.
For the reason described above, to achieve a thinner fluororesin layer is pursued daily. Recently, a fluororesin tube having a thickness of approximately 30 microns has been developed.
If the fixing roller or the fixing sleeve formed of a rubber layer covered with a fluororesin tube is used for a long period of time, a minute crack may occur on a surface of the fluororesin layer (hereinafter, this crack is referred to as “crack in the fluororesin layer”). The crack in the fluororesin layer may cause an image defect when the toner image is fixed. In addition, if being further used, the fluororesin layer may rupture and drop out from the surface of the rubber layer, which may cause a problem that the fixing roller and the fixing sleeve cannot be used any more.
As a countermeasure of the crack in the fluororesin layer, a method of improving crack resistance property of the fluororesin is proposed. Patent Document 1 proposes a method of using a “copolymer of tetrafluoroethylene and perfluoromethylvinylether”, or a “copolymer of tetrafluoroethylene and perfluoroethylvinylether” for the fluororesin. Patent Document 2 proposes a fixing apparatus in which a surface parting layer contains a copolymer of tetrafluoroethylene and perfluoroethoxyethylene, and transmittance of hydrochloric acid is 2.0×10−5 g·cm/cm2 or lower.    Patent Document 1: Japanese Patent Application Laid-Open No. H09-011362    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-126576